/Engine/Source/Runtime/Slate/Public/Framework/Application/SlateApplication.h
Delegate called just before possible focus change
typedef TMulticastDelegate_FiveParams< void, const [FFocusEvent](API\Runtime\SlateCore\Input\FFocusEvent) &, const [FWeakWidgetPath](API\Runtime\SlateCore\Layout\FWeakWidgetPath) &, const [TSharedPtr](API\Runtime\Core\Templates\TSharedPtr)< [SWidget](API\Runtime\SlateCore\Widgets\SWidget) > &, const [FWidgetPath](API\Runtime\SlateCore\Layout\FWidgetPath) &, const [TSharedPtr](API\Runtime\Core\Templates\TSharedPtr)< [SWidget](API\Runtime\SlateCore\Widgets\SWidget) > & > FOnFocusChanging